This invention relates to an ignition timing control system of the type that employs a capacitative-discharge ignition unit (commonly referred to as a C.D.I unit) for use in internal combustion engines.
In general, a rise in the rotational speed of a two-cycle engine causes the mixture introduced into an engine cylinders to develop intake inertia. When an exhaust port opens under such a condition, the efficiency at which the mixture is charged into the cylinder apparently rises, thus elevating the cylinder internal pressure, owing to the effect that the cylinder internal pressure and the reflected exhaust wave resonate. Engine power can be improved if the ignition timing is controlled so as lag by a predetermined angle in relation to the rise in internal cylinder pressure so that firing occurs at the instant the cylinder internal pressure assumes a peak value.
In order to improve the power characteristic in an intermediate range of engine speeds in a two-cycle engine wherein greater engine power at high engine speed is achieved by delaying the ignition timing, it is effective to modify the shape and/or dimensions of the engine expansion chamber, as by being increased in length. However, the increase in length can cause the resonance point of reflected exhaust wave internally of the expansion chamber to shift toward the low engine rpm side. As a result, a satisfactory mixture charging efficiency can no longer be obtained at high engine speed, thus diminishing engine power at high rpm. Since the conventional ignition timing control system for two-cycle engines has but a simple delay angle characteristic, compensation for this decline in engine power at high engine speed cannot be achieved.